1. Field of the Invention
The present invention relates to a light scanning apparatus which irradiates an image bearing member with a light beam in an image forming apparatus.
2. Description of the Related Art
In a conventional electrophotographic image forming apparatus such as a copying machine and a printer, a light scanning apparatus irradiates a surface of an image bearing member with a light beam to form an electrostatic latent image on the image bearing member. The electrostatic latent image thus formed is developed into a toner image with developer (toner), and the toner image is transferred onto a recording medium. In this manner, an image is formed.
The light scanning apparatus provided in the conventional electrophotographic image forming apparatus includes a light source configured to emit a light beam based on image information, and a light deflector configured to deflect the light beam for scanning. Further, the light scanning apparatus includes an fθ lens configured to scan, at constant speed, the light beam which is deflected by the light deflector, and to image the light beam on the image bearing member, a mirror configured to reflect the light beam in a predetermined direction, and a housing configured to hold those optical members.
The image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-298243 is intended to reduce noise or the like generated by excitation sound of a motor of the light deflector inside the light scanning apparatus. Therefore, Japanese Patent Application Laid-Open No. 2000-298243 discloses a configuration in which an opposing wall surface which reflects excitation sound is provided so that a difference between a direct propagation distance of the excitation sound from an outer circumferential portion of a stator coil to a cover member of the housing and a reflective propagation distance from the outer circumferential portion of the stator coil to the cover member via the opposing wall surface becomes a half wavelength of the excitation sound. Further, Japanese Patent Application Laid-Open No. 2000-298243 discloses a configuration in which a recessed portion is provided as the opposing wall surface in a bottom plate of the housing. In the light scanning apparatus of the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-298243, in order to prevent an obstacle from being located between the light deflector and the opposing wall surface, the outer circumferential portion of the stator coil is arranged outside a region immediately above a motor drive circuit board. That is, the recessed portion as the opposing wall surface is arranged outside a region immediately below the motor drive circuit board.
The light deflector rotates to generate heat, and the heat thus generated is transferred to the housing of the light scanning apparatus. However, the conventional technology as disclosed in Japanese Patent Application Laid-Open No. 2000-298243 does not take into consideration a thermal stress generated in the housing and the motor drive circuit board (drive circuit board) of the light deflector mounted on the housing, due to a difference in coefficient of linear expansion of materials forming the housing and the motor drive circuit board, caused by the generated heat. This thermal stress causes thermal deformation in the housing and the motor drive circuit board, and as a result, the postures of the optical members such as the light deflector, lenses, and mirrors held in the housing become unstable. Such instability causes increase in spot diameter of the light beam on the surface of the image bearing member and shift of an irradiation position of the light beam. As a result, color misregistration occurs in the image formed on the sheet, leading to image degradation.
The light deflector is positioned with respect to the housing by fitting a bearing portion of the light deflector into a bearing holding portion of the housing. The light deflector is mounted on the housing by a fixing portion provided in the bottom plate of the housing. The heat generation source of the light deflector is the bearing portion which supports a rotary shaft of the motor. The heat generated in the bearing portion of the light deflector is transferred along a path from the bearing portion to the bearing holding portion of the housing, in turn from the bearing holding portion to the bottom plate of the housing, and in turn from the bottom plate to the fixing portion. Such heat transfer causes thermal deformation in the housing. On the other hand, the bearing portion is further supported by the drive circuit board of the light deflector, and the drive circuit board is fixed to the fixing portion of the housing. The heat generated in the bearing portion is transferred also to the drive circuit board. Such heat transfer causes thermal deformation in the drive circuit board. In general, the housing and the drive circuit board are made of different materials, and hence the thermal deformation amount of the housing and the thermal deformation amount of the drive circuit board are different from each other due to the difference in coefficient of linear expansion between the housing and the drive circuit board. Due to the difference in thermal deformation amount, the thermal stress is generated in the housing and the drive circuit board. Therefore, it is necessary to suppress the thermal stress between the bearing holding portion and the fixing portion.